Method and coating system for reducing carbonaceous deposits on surfaces exposed to hydrocarbon fuels at elevated temperatures

ABSTRACT

A coating system and method for reducing the tendency for hydrocarbon fluids, such as fuels and oils, to form carbonaceous deposits that adhere to the walls of a containment article. Of particular concern are carbonaceous deposits that form at temperatures below about 650° F. (about 345° C.). The coating system combines an outermost layer of platinum with a ceramic barrier layer. The coating system has been shown to significantly reduce the formation of carbonaceous deposits at temperatures between about 220° F. and 650° F. (about 105° C. to about 345° C.), as well as reduce the adhesion of such deposits. The platinum outermost layer also serves as a radiation shield to reduce heat transfer from the containment article to the hydrocarbon fluid. The outermost layer is preferably deposited as an extremely thin film by chemical vapor deposition. The barrier layer is deposited to a thickness sufficient to prevent interdiffusion of the platinum outermost layer with the containment wall.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to coatings that inhibit theformation and adhesion of deposits on surfaces that contact hydrocarbonfluids. More particularly, this invention relates to a method andcoating system for preventing or reducing the deposition of carbonaceousdeposits on surfaces that contact a hydrocarbon fluid at hightemperatures.

[0003] 2. Description of the Related Art

[0004] Thermal instability, or in the case of fuels, fuel instability,generally refers to the formation of undesired deposits that occurs whenhydrocarbon fluids, such as fuels and lubricating oils, are at hightemperatures, generally above about 140° C. In the case of fuels, it isgenerally accepted that there are two distinct mechanisms occurringwithin two overlapping temperature ranges. In the first mechanism,referred to as the coking process, a generally consistent increase inthe rate of formation of coke deposits occurs above temperatures ofabout 650° F. (about 345° C.). Coke formation is the result of highlevels of hydrocarbon pyrolysis, and eventually limits the usefulness ofthe fuel. A second mechanism primarily occurs at lower temperatures,generally in the range of about 220° F. to about 650° F. (about 105° C.to about 345° C.), and involves oxidation reactions that lead topolymerization and carbonaceous gum deposits.

[0005] In the past, the solution to the problem of gum and cokeformation was primarily directed toward placing limitations on fuelchemistry and impurities associated with fuels, as disclosed in U.S.Pat. Nos. 2,698,512, 2,959,915 and 3,173,247. However, the propensityfor gum and coke formation is increased with certain hydrocarbon fluidsfor fuels, oils, lubricants, petrochemical processes (plastics andsynthetics) and the like, especially those derived from nonpetroleumsources, such as shale and coal, which can exhibit significantly moreproblems with thermal instability because of their high content ofolefins, sulfur and other compounds. The consequences of thermalinstability and fuel instability are of even greater significance withdeveloping technology that requires processes and machinery to operateat higher temperatures, as afforded by advances in materials technology.Accordingly, fluid containment articles that are resistant to or preventthe formation of adverse decomposition products and foulants arenecessary in applications where thermal instability, including fuelinstability, is a problem as a result of exposure to such fluids to hightemperatures. Particularly notable applications include thefuel-handling and hydraulic components of gas turbine engines. With theadvent of higher engine operation temperatures and the use of fuel as aheat sink, there is an increased likelihood that fluid flow through suchcomponents will be restricted or even blocked by carbonaceous deposits.

[0006] It has been recognized that deposits can form as a result of areaction between a hydrocarbon fluid and its containment wall. In U.S.Pat. No. 4,078,604, heat exchangers are provided with thin-walledcorrosion-resistant layers of electrodeposited gold or similarcorrosion-resistant metals on the walls of the cooling channels in orderto make the surfaces corrosion resistant to such storable liquid fuelsas red fuming nitric acid. In this case, the wall is protected fromcorrosion, and the intent is not to prevent deposit formations.Furthermore, gold readily diffuses into other materials at elevatedtemperatures, and therefore is unsuitable as a protective coating forhigh temperature applications, e.g., temperatures associated with gumand coke formation.

[0007] More recently, coating systems specifically directed toinhibiting the formation and adhesion of carbonaceous deposits have beentaught. For example, U.S. Pat. Nos. 5,805,973, 5,891,584 and 5,923,944and U.S. patent application Ser. No. 09/955,043, all assigned to theassignee of the present invention and incorporated herein by reference,teach the use of coke barrier coatings (CBCs) that eliminate or modifythe surface reactions which lead to formation of thermal instabilitydeposits from hydrocarbon fluids, and reduce adhesion of such deposits.These patents are generally directed to ceramic coatings that areespecially capable of reducing deposits at very high temperatures, e.g.,above 650° F. (about 345° C.). As an example, U.S. Pat. Nos. 5,805,973and 5,891,584 disclose coatings that catalyze thermal decomposition inthe hydrocarbon fluid to actually promote the formation of coke, whichis substantially nonadherent to the coatings.

[0008] Many applications exist where there is a particular need forcoatings that can significantly reduce the formation and adhesion ofcarbonaceous deposits at lower temperatures, such as fuel/air heatexchangers, fuel nozzles, oil sumps and other fuel and hydraulic systemcomponents of gas turbine engines. For this type of hardware, reductionsin hydrocarbon deposits have been achieved with the use of coatings thatare not reactive with hydrocarbons. In situations where heat transferfrom the containment walls is a major contributor to the fluidtemperature, thermally-reflective (low emissivity) coatings that reduceheat transfer to the hydrocarbon fluid have been employed to reducedeposit formation. Notably, the CBC systems taught by U.S. Pat. Nos.5,805,973, 5,891,584 and 5,923,944 and U.S. patent application Ser. No.09/955,043 do not have the correct optical properties, including lowemissivity, to function as radiation shields. While CBC systems of theprior art can be combined with low-emissivity coatings, a significantdrawback is the additional volume, weight and cost incurred.Accordingly, it would be desirable if an improved coating system wereavailable that reduced the formation of carbonaceous deposits inhydrocarbon fluids at temperatures below about 650° F., reduced theadhesion of such deposits, and reduced the temperature of thehydrocarbon fluids.

SUMMARY OF INVENTION

[0009] The present invention provides a coating system and method forreducing the tendency for hydrocarbon fluids, such as fuels and oils, toform carbonaceous deposits that adhere to their containment surfaces.The invention is particularly concerned with the carbonaceous depositsthat form at temperatures below about 650° F. (about 345° C.). Accordingto the invention, a coating system that combines an outermost layer ofplatinum with a ceramic barrier layer has been shown to significantlyreduce the formation of carbonaceous deposits at temperatures betweenabout 220° F. and 650° F. (about 105° C. to about 345° C.), as well asreduce the adhesion of such deposits. The platinum outermost layer alsoserves as a radiation shield to reduce heat transfer from thecontainment article to the hydrocarbon fluid. The outermost layer ispreferably deposited as an extremely thin film by chemical vapordeposition. The barrier layer is deposited to a thickness sufficient toprevent interdiffusion of the platinum outermost layer with the wall onwhich the platinum outermost layer is deposited.

[0010] From the above, it can be seen that the coating system of thisinvention can be present as a very thin coating, yet performs threedistinct functions that reduce hydrocarbon fluid deposits attemperatures below about 650° F. Accordingly, the coating system of thisinvention does not share the disadvantages of volume, weight and costnoted for prior attempts to combine CBCs with low-emissivity coatings.

[0011] Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 represents a cross-sectional view of a containment wallhaving a coating system in accordance with this invention.

[0013]FIGS. 2 and 3 are graphs that illustrate reduced deposition ratesachieved with the coating system shown in FIG. 1 as compared to priorart coating systems.

[0014]FIG. 4 is a graph representing incident radiant energy for a fuelinjector over a wavelength band of 1 to 7 micrometers.

[0015]FIG. 5 is a graph showing the reflectance versus wavelength plotfor the coating system of FIG. 1 over that portion of the wavelengthband shown in FIG. 4 with the highest incident radiant energy.

DETAILED DESCRIPTION

[0016]FIG. 1 represents a coating system 14 for a component 10 with acontainment wall 12 that contacts a hydrocarbon fluid (e.g., fuels andoils) at elevated temperatures. The coating system 14 serves to preventor at least significantly reduce the formation and adhesion ofcarbonaceous deposits that would otherwise adhere to the wall 12 ifmaintained at a temperature of up to about 650° F. (about 345° C.). Theinvention is applicable to any hydrocarbon fluid in which carbonaceousgum (or other polymers) deposits form when the fluid is subjected toelevated temperatures, generally above 140° C. and particular attemperatures of about 220° F. to 650° F. (about 105° C. to 345° C.).Such fluids may be pure hydrocarbon or mixtures thereof. Fluidcontainment articles that can benefit from the present invention may beany component which is adapted to contain or transport hot hydrocarbonfluid, and include but are not limited to fuel nozzles, pipes, oil sumpsand heat exchangers of gas turbine engines. With each of these examples,the containment walls of the component typically transfer heat from anexternal heat source to the hydrocarbon fluid within the component.

[0017] In the example represented in FIG. 1, a liquid hydrocarbon fluid(not shown) contacts and flows across the surface of the wall 12protected by the coating system 14, such that heat transferred to thefluid from an external heat source must be conducted through the coatingsystem 14. Accordingly, the wall 12 is protected by the coating system14 which, in accordance with this invention, reduces or prevents theformation and adhesion of carbonaceous deposits from the fuel that tendto occur as a result of the elevated temperatures of the wall 12 andfuel. The wall 12 may generally be constructed of any suitable materialfor the particular application. Typical materials include stainlesssteel, corrosion-resistant alloys of nickel and chromium, andhigh-strength nickel-base alloys. Notably, alloys such as these thatcontain iron, chromium and nickel appear to cause or promote theformation of fuel thermal decomposition products such as gum and coke inliquid hydrocarbon fluids and fuels.

[0018] The coating system 14 of this invention includes a low-emissivityplatinum layer 18 and a diffusion barrier layer 16 that separates theplatinum layer 18 from the wall 12 of the component 10.Though shown asconsisting of only two layers, it is foreseeable that additional coatinglayers could be employed. The coating system 14 is preferably continuousand completely covers all surfaces of the wall 12 that would otherwisecontact the fuel.

[0019] In accordance with the present invention, the platinum layer 18of the coating system 14 is smooth, is reactive with hydrocarbon fluidsat elevated temperatures and exhibits low emissivity toward thehydrocarbon fluid contained by the wall 12. The platinum layer 18exhibits sufficiently low emissivity so that radiation heat transfer tothe hydrocarbon fluid is reduced. As such, the temperature of the fluid,and therefore the tendency for the fluid to form carbonaceous deposits,is reduced. Emissivity values (ε) of about 0.2 or less are believed tobe suitable for purposes of this invention. To promote emissivity, apreferred surface roughness for the platinum layer 18 is about 40microinches (about 1.0 micrometer) R_(a) or less. This aspect of thecoating system 14 also reduces the amount of time that the bulk fluidhas to react near the coating surface within the fluid boundary layer,reducing both surface reaction time and concentration of depositprecursors (radicals and atoms) that provide for polymer growth.

[0020] According to this invention, hydrocarbon fluid that eventuallybecomes sufficiently hot to form carbonaceous gum deposits is catalyzedby the platinum layer 18 to promote the rapid formation of gumsubstances. It is believed that the platinum layer 18 of this inventioncatalyzes the formation of carbonaceous gum substances in a hydrocarbonfluid to the extent that, in a flowing fuel system, the gum substancesgrow too quickly to allow them to adhere to the wall 12. Instead, gumsubstances are found in the form of very fine particulate within thefluid.

[0021] The thickness of the platinum layer 18 should generally take intoaccount the growth properties of platinum as it is deposited, as well asthe surface roughness of the wall 12. A suitable thickness for theplatinum layer 18 is about 100 to about 500 nanometers, with a morepreferred range being about 150 to about 200 nanometers. In a preferredembodiment, the platinum layer 18 continuously covers the surface of thewall 12 to provide the desired chemical and reflective propertiesthroughout the flow system where elevated wall and/or fluid temperaturesare lilikely

[0022] The diffusion barrier layer 16 prevents interdiffusion betweenthe platinum layer 18 and the article wall 12, which would occur at anunacceptable rate at the temperatures of concern for the invention. Thebarrier layer 16 also protects the wall 12 from chemical attack fromcontaminants in the fuel, such as sulfur and water that would formsulfuric acid and pit the surface of the wall 12. Therefore, with theprotective barrier layer 16, the coating system 14 prevents or inhibitsreactions between constituents of the fuel and wall 12. Preferredmaterials for the barrier layer 16 include ceramics such as silica(SiO₂) and alumina (Al₂O₃), though other ceramics could be used,including yttria (Y₂O₃), hafnia (HfO₂), tantala (Ta₂O₅), mullite(3Al₂O₃2SiO₂), and complex chemical combinations of silica with boronand/or phosphorous and/or alumina. As previously noted, the thickness ofthe barrier layer 16 must be sufficient to prevent interdiffusion withthe material of the article wall 12. While optimal thicknesses willdepend in part on the composition of the barrier layer 16, a suitablethickness range is about 500 to about 1500 nanometers, with a morepreferred range being about 700 to about 1300 nanometers.

[0023] According to this invention, coatings having the above-describedcharacteristics serve to prevent or at least considerably reduce theformation, deposition and adhesion of carbonaceous gum and otherdecomposition impurities. As evident from the above, a requirement forthe low-emissivity coating system 14 of this invention is for thebarrier and platinum layers 16 and 18 to be deposited in such a manneras to obtain a suitable surface smoothness. According to the invention,a preferred deposition method is chemical vapor deposition (CVD), whichis able to deposit the layers 16 and 18 on the wall 12 so that thesurface finish of the coating system 14 replicates that of theunderlying wall 12.

[0024] In an investigation leading to this invention, a suitableplatinum layer 18 was deposited by CVD using platinum acetyl acetone(Et₂(PtOAc)₂) as the chemical precursor, and with the followingdeposition parameters: deposition temperature of about 440° C.,deposition pressure of about 500 mtorr, and a duration of about sixtyminutes. The resulting platinum layer had a thickness of about 200nanometers. Prior to depositing the platinum layer, a barrier layer 16of silica was deposited by CVD at a temperature of about 700° C. andpressure of about 500 mtorr, over a period of about two hours.

[0025]FIG. 4 is a graph representing incident radiant energy for a fuelinjector of a low-emission combustion system. Incident radiant energy isshown as peaking within a wavelength band of about 1 to 7 micrometers.In FIG. 5, the reflectance versus wavelength plot for a coating system14 in accordance with the embodiment of FIG. 1 is shown over thatportion of the wavelength band shown in FIG. 4 with the highest incidentradiant energy. FIG. 5 illustrates that the CVD platinum layer 18 ofthis invention exhibits high reflectivity (low emissivity) over thecritical wavelengths of about 2 to 6 micrometers. Accordingly, withinthe operating environment of the fuel injector, the platinum layer 18 ofthis invention is very effective in reducing radiation heat transferfrom the surface on which the layer 18 is deposited to the fluidcontacting the layer 18.

[0026] In another investigation leading to this invention, depositionrates were determined for tube specimens formed of Inconel 625, Inconel718, 321 stainless steel (SS), or 347SS. Jet-A fuel was flowed throughthese specimens at a pressure of about 500 psi (about 345 bar) and at aflow rate of either about 10 ml/minute for about 150 hours or about 150ml/minute for about 100 hours. The specimens were placed in a furnacewhere their external surfaces were heated to a temperature of about 500°C. The fuel was heated by the tube walls of the specimens, so that fueltemperature increased as the fuel flowed further through the length ofeach tube (the abscissa of FIG. 2). Testing was performed with fiftyuncoated specimens and fifty specimens protected by the coating systemof this invention. Each coating system included a platinum layer ofabout 150 nanometers in thickness separated from the internal tube wallsby a silica barrier layer having a thickness of about 500 to 600nanometers. At the completion of the 150 hour test, the amount ofcarbonaceous deposits was measured for each specimen relative tolocation along the lengths of the specimens. From FIG. 2, it can be seenthat very little deposition occurred on those specimens protected by thecoating system of this invention, while uncoated specimens experiencedhigh deposition rates on wall portions of the specimens that sustainedfuel temperatures above about 350° C.

[0027] A final evaluation was then performed to compare the performanceof the coating system of this invention against different coatingcompositions. Deposition rates were again determined for twenty-fivetube specimens formed of formed of Inconel 625, Inconel 718, 321SS or347SS and under the same conditions as described above. Testing wasperformed on ten of each of the following specimens. TABLE I CoatingMaterials Specimen Outer/Barrier CBC-A CVD tantala/CVD silica CBC-B CVDsilica CBC-C CVD zirconia/CVD silica CBC-D CVD Pt/CVD silica

[0028] Coating thicknesses were about 150 to 200 nm for the outer layers(tantala, zirconia, platinum), and about 700 to 1300 nm for the barrierlayers (silica). The total coating thicknesses of the all-silica CBC-Bspecimens were approximately the same as the coating thicknesses for theother specimens. As with the previous test, at the completion of about150 hours, the amount of carbonaceous deposits was measured for eachspecimen relative to location along the lengths of the specimens. FromFIG. 3, it can be seen that very little deposition occurred on thespecimens protected by the coating system of this invention (CBC-D). Incontrast, those specimens coated with the other evaluated coatingsexperienced significantly higher deposition rates on specimen walls thatsustained fuel temperatures of about 250° C. to about 650° C.

[0029] While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

1. A hydrocarbon fluid containment article comprising a wall and acoating system on the wall, the coating system comprising an outermostlayer of platinum and a ceramic barrier layer between the outermostlayer and the wall:
 2. The hydrocarbon fluid containment articleaccording to claim 1, wherein the outermost layer has a thickness ofabout 150 to about 200 nm.
 3. The hydrocarbon fluid containment articleaccording to claim 1, wherein the barrier layer has a thickness of about500 to about 1500 nm.
 4. The hydrocarbon fluid containment articleaccording to claim 1, wherein the barrier layer is formed of at leastone ceramic material chosen from the group consisting of silica,alumina, tantala, hafnia, yttria, and chemical combinations of silicawith boron and/or phosphorous and/or alumina.
 5. The hydrocarbon fluidcontainment article according to claim 1, wherein the outermost layercontacts a hydrocarbon fluid at a temperature of about 105° C. to about345° C.
 6. An article having a wall contacting a hydrocarbon fluid at atemperature of about 105° C. to about 345° C., the article comprising acoating system on the wall that inhibits the formation and adhesion ofcarbonaceous deposits on the wall, the coating system comprising anoutermost layer of platinum and a ceramic barrier layer between theoutermost layer and the wall, the outermost layer having a thickness ofabout 150 to about 500 nm, the barrier layer having a thickness of about500 to about 1500 nm, the barrier layer being formed of at least oneceramic material chosen from the group consisting of silica and alumina.7. The article according to claim 6, wherein the outermost layer has athickness of about 150 to about 200 nm.
 8. e article according to claim6, wherein the barrier layer has a thickness of about 700 to about 1300nm.
 9. The article according to claim 6, wherein the barrier layer isformed of either silica or alumina.
 10. The article according to claim6, wherein the article is a gas turbine engine component.
 11. Thearticle according to claim 6, wherein the article is a gas turbineengine component chosen from the group consisting of fuel/air heatexchangers, pipes, fuel nozzles and oil sumps.
 12. The article accordingto claim 6, wherein the outermost layer and the barrier layer aredeposited by chemical vapor deposition.
 13. The article according toclaim 6, wherein the outermost layer has a surface roughness of notgreater than about one micrometer R_(a). The article according to claim6, wherein the outermost layer has a surface roughness of not greaterthan about one micrometer R_(a).
 14. A method for inhibiting theformation and adhesion of carbonaceous deposits on a wall of ahydrocarbon fluid containment article, the method comprising the step ofdepositing a ceramic barrier layer on the wall, and then depositing anoutermost layer of platinum on the ceramic barrier layer.
 15. The methodaccording to claim 14, wherein the outermost layer has a thickness ofabout 150 to about 200 nm.
 16. The method according to claim 14, whereinthe barrier layer has a thickness of about 500 to about 1500 nm.
 17. Themethod according to claim 14, wherein the barrier layer is formed of atleast one ceramic material chosen from the group consisting of silica,alumina, tantala, hafnia, yttria, and chemical combinations of silicawith boron and/or phosphorous and/or alumina.
 18. The method accordingto claim 14, further comprising the step of contacting the outermostlayer with a hydrocarbon fluid at a temperature of about 105° C. toabout 345° C.
 19. The method according to claim 14, wherein theoutermost layer has a thickness of about 150 to about 200 nm and thebarrier layer has a thickness of about 700 to about 1300 nm.
 20. Themethod according to claim 14, wherein the article is a gas turbineengine component.